The production of cement generates substantial quantities of carbon dioxide. The carbon dioxide is produced in two ways. One of the materials in cement production is calcium carbonate. The calcium carbonate is heated to calcine it and loses carbon dioxide to form calcium oxide. The burning of carbon-containing fuel to generate the high temperatures required to produce cement also generates carbon dioxide. For environmental reasons it is desirable to reduce the amount of carbon dioxide released into the atmosphere.
The raw materials for cement production, which include calcium carbonate, are generally mixed, dried and ground to form a “raw meal” which is then processed to produce clinker, the main component of cement. In a preheating step the raw meal is heated to just below calcination temperature. It then passes to a calciner where further heating takes place and, as the temperature rises, calcination of the calcium carbonate to calcium oxide takes place and carbon dioxide is released. The calcined raw meal then passes to a kiln where it is heated to higher temperatures to produce clinker which is subsequently ground to yield cement powder.
Air is generally used as a source of oxygen for combustion of fuel to provide heat for the calciner and the kiln. The principal gases present in air are nitrogen (about 80% by volume) and oxygen (about 20% by volume).
Combustion of a solid or liquid, carbon-based fuel can be summarised in the equation:C+O2=CO2 
The volume of the solid or liquid fuel is negligible relative to that of the gases involved. One volume of oxygen produces one volume of carbon dioxide. Nitrogen present in the air used for combustion is not involved in the combustion. Following the combustion the gas stream contains about 80% by volume of nitrogen and about 20% of carbon dioxide. The overall volume of gas is increased by the heat generated. When a solid or liquid hydrocarbonaceous fuel is burned combined hydrogen present in the fuel combines with oxygen according to the equation:4H+O2=2H2O
The water generated is in the gaseous state and is therefore present as a gaseous diluent of the carbon dioxide which is also produced. One volume of oxygen produces two volumes of gaseous water, the volume of the solid or liquid hydrocarbonaceous fuel being negligible relative to the volumes of gas involved. Gaseous water therefore contributes to an increase in the volume of gas formed by combustion compared to the volume of oxygen used. It is, however, readily separated from the carbon dioxide by simple cooling at a later stage in the operation. The overall volume of gas increases at the combustion stage with the increased temperature.
In the combined gases from the calcination process and from fuel combustion the carbon dioxide is generally present at from 14 to 33% by volume. The other principal constituent of the gas is nitrogen. Although the mixture of nitrogen and carbon dioxide could be separated and the carbon dioxide stored to prevent its release into the atmosphere the energy required to effect the separation is too high to make it worthwhile. If the nitrogen in air is removed and the resulting substantially pure oxygen used to burn a fuel, nitrogen is then not present as a diluent of the carbon dioxide produced.
If oxygen were used instead of air in burning all of the fuel required to produce cement additional cost might be incurred. In addition, combustion processes involving undiluted oxygen generally proceed with great vigour and generate very high temperatures capable of damaging or shortening the working life of production plant.